1. Field of the Invention
The present invention relates to a toner for developing an electrostatic latent image in electrophotography, electrostatic recording, and electrostatic printing. In addition, the present invention also relates to a developer, a toner container, a process cartridge, an image forming method, and an image forming apparatus using the toner.
2. Discussion of the Background
In an image forming process such as electrophotography, electrostatic recording, and electrostatic printing, first, an electrostatic latent image is formed on an electrostatic latent image bearing member (hereinafter “electrophotographic photoreceptor” or “photoreceptor”). The electrostatic latent image is then formed into a visible toner image by a developer. The toner image is transferred onto a recording medium such as paper, and finally fixed thereon.
The developer for developing an electrostatic latent image includes one-component developer consisting essentially of magnetic or non-magnetic toner, and two-component developer including toner and carrier.
To fix a toner image on a recording medium in electrophotography, a heating roller method in which a heating roller is directly pressed against the toner image on the recording medium is widely used because of its high energy efficiency. However, the heating roller method requires a great amount of electric power, which is disadvantageous from the viewpoint of energy saving. To overcome such a disadvantage, various attempts have been made to reduce electric power consumption of the heating roller. One proposed approach involves decreasing output of the heating roller while image formation is not occurring (i.e., sleep mode) and increasing it during image formation so that the temperature of the heating roller is increased.
However, the above approach requires several ten seconds until the heating roller recovers from the sleep mode and is heated to an appropriate temperature for fixing, which may cause stress to users. On the other hand, the heating roller is preferably turned off completely while image formation is not occurring so that electric power consumption is reduced. Accordingly, toner is required to be fixable even when the temperature of the heating roller is low.
As described above, recent toners are required to be fixable at low temperatures (this property is hereinafter referred to as low-temperature fixability) and have good heat-resistant storage stability as well. To respond to these requirements, recently polyester resins are used as binder resins because of their excellent low-temperature fixability and affinity for paper, in place of styrene resins that are widely used as binder resins conventionally. For example, Unexamined Japanese Patent Application Publication No. (hereinafter “JP-A-”) 2004-245854 discloses a toner including a linear polyester resin having specific properties such as a specific molecular weight. JP-A-04-70765 discloses a toner including a non-linear cross-linked polyester resin formed from a rosin which serves as an acid component.
However, these toners do not function in recent high-speed and energy-saving electrophotographic image forming apparatuses, possibly providing weak fixation strength due to shortening of the fixing time and lowering of the fixing temperature.
It should be noted that the polyester resin formed from a rosin, which is disclosed in JP-A-04-70765, is easily pulverized, resulting in good toner manufacturability in pulverization methods. Further, the polyester resin includes 1,2-propanediol, which is a branched-chain alcohol having 3 carbon atoms, as an alcohol component, which provides better low-temperature fixability without decreasing hot offset temperature compared to resins including an alcohol having 2 carbon atoms or less. The “hot offset” here refers to an undesirable phenomenon in which part of a fused toner image is adhered to the surface of a heat member, and re-transferred onto an undesired portion of a recording medium. In addition, such a polyester resin including a branched-chain alcohol having 3 carbon atoms is able to prevent deterioration of heat-resistant storage stability even when the glass transition temperature is decreased, while a resin including a branched-chain alcohol having 4 carbon atoms or more is not. Accordingly, such a polyester resin is suitable as a binder resin from the viewpoint of low-temperature fixability and heat-resistant storage stability.
However, such a polyester resin will be not able to respond to severe requirements for energy-saving in the near future.
To overcome such a situation, JP-A-2006-208609 discloses a toner including a fixing auxiliary component. The fixing auxiliary component forms crystalline domains thereof in the toner so that the toner provides both heat-resistant storage stability and low-temperature fixability simultaneously. However, this attempt may not be sufficient to respond to the severe requirements for energy-saving.